Update on Mar 10, 2014 by
Bishop Hill
Bishop Hill
Nic Lewis has a post up at Climate Audit, looking at the new paper by Gavin Schmidt's colleague Drew Shindell.
Shindell, the lone author of the paper, looks at CMIP5 models and claims to show that there are distinct differences between the climate's sensitivity to different forcings. Once these are taken into account, and once a lot of adjustments are made to them too, it is possible to show (allegedly) that low climate sensitivity to carbon dioxide is not possible.
These adjustments are not trivial, as Lewis explains:
One of those adjustments is to add +0.3 W/m² to the figures used for model aerosol forcing to bring the estimated model aerosol forcing into line with the AR5 best estimate of -0.9 W/m². He notes that the study’s main results are very sensitive to the magnitude of this adjustment. If it were removed, the estimated mean TCR would increase by 0.7°C. If it were increased by 0.15 W/m², presumably the mean TCR estimate of 1.7°C would fall to 1.35°C – in line with the Otto et al (2013) estimate. Now, so far as I know, model aerosol forcing values are generally for the change from the 1850s, or thereabouts, to ~2000, not – as is the AR5 estimate – for the change from 1750. Since the AR5 aerosol forcing best estimate for the 1850s was -0.19 W/m², the adjustment required to bring the aerosol forcing estimates for the models into line with the AR5 best estimate is ~0.49 W/m², not ~0.3 W/m². On the face of it, using that adjustment would bring Shindell’s TCR estimate down to around 1.26°C.
The analysis also relies on the CMIP5 models' representation of climate and the various forcings being realistic, and Lewis has taken a detailed look at the individual models and the multimodel mean. These do not exactly encourage confidence. The scaling factor - the amount by which you have to alter the model estimate to get a match with the observations is of the order of 70%. When you look at the individual models it's even worse:
To summarise, four out of six models/model-averages used by Shindell are included...in AR5 Figure 10.4 ... none of these show scaling factors for ‘other anthropogenic’...that are consistent with unity at a 95% confidence level. In a nutshell, these models at least do not realistically simulate the response of surface temperatures and other variables to these factors.
It's rather amusing really. Read the whole thing.
Bishop Hill